Numerical Study of Compressive Behavior of Concrete at High Strain Rates

High strain rate unconfined compressional tests on concrete are simulated by a 3D discrete-element method. The laboratory data set was provided by three unconfined experiments on a split Hopkinson pressure bar apparatus at very high strain rates (350–700 s?1). This numerical method was chosen because it is well adapted to problems involving the characterization of fracturing and fragmentation in geomaterials. The simulations input data are the recorded experimental velocities, whereas the simulations output data are the computed forces that are compared with the experimental ones. The fit between the experimental and the numerical data is quite good. Based on this fit, it is shown that the strain rate dependency of the material strength can be explained by inertial effects.     

Effect of Specimen Diameter in Compression at High Strain Rates

The split Hopkinson pressure bar (SHPB) is one of the most widely used experimental techniques to study the behavior of materials at medium to high strain rates. This paper deals with the use of the SHPB to investigate the effect of the specimen diameter on the mechanical properties of materials. Specimens made of aluminum alloy 6061-T6, with length 5?mm and varying diameters of 5, 6, 8, 10, and 12?mm, are tested. For same strain rate, higher stresses are observed for the smaller-diameter specimens. The smaller-diameter specimens give rise to higher and more uniform strain rates but oscillating stress-strain curves. They result in a low transmission coefficient that affects the output signals; therefore, smaller diameters should be selected carefully with the transmission coefficient of 0.5 or better. With some compromise on oscillating stress-strain behavior, the specimens with smaller diameters can be used successfully to achieve high strain rates.     

Microstructure-Property Relationship of High Strength Low Alloy Steels During Tensile Testing at High Strain Rates

The influence of strain-rate on the room temperature mechanical properties of Dual-Phase and Transformation Induced Plasticity (TRIP) steels was investigated.The results showed that both plastic strain,and strength properties increased with increasing strain rates at high strain rates.At strain rates lower than approximateil 1s-1 the properties no longer have an advantageous proportionality to strain rate and remain strain rate neutral.Possible explanations are offered for trends exhibited,in terms of thermal and athermal considerations,in relation to the respective microstructures of the two steels.     

不同应变速率下TiAl基合金压缩断裂行为的研究

通过力学性能测试和扫描电镜观察,研究了全层组织和双态组织γ-TiAl基合金在不同应变速率下的压缩断裂行为。结果表明：室温下其压缩性能远远优于拉伸性能,且表现出一定的塑性;材料的力学性能参数对应变速率非常敏感,当应变速率逐渐增加时,全层组织的屈服强度和抗压强度均呈增大趋势;而双态组织在应变速率由静态、准静态到动态的变化范围内,屈服强度和抗压强度的值先减小后增大,当应变速率ε′=9.8×10^-4s^-1时,屈服强度和抗压强度的值达到最小;试样的最终断裂是通过裂纹的形核、扩展以及相互贯通而形成的,断裂面主要由剪应力形成的剪切无定形断裂特征和正应力形成的解理断裂特征组成,并且在不同应变速率下其断口也呈现出规律性的变化。     

高应变率下的岩石动载特性

采用先进的霍布金生杆(SHPB)装置对多种矿岩进行了高应变率下(ε=300～1000s^-1)的力学特性研究。给出了矿岩在不同应变率下的动态破坏强度、动态弹性模量、动态应力-应变曲线和分析结论。     

高性能纳米晶镨铁硼稀土永磁材料的研究现状

讨论了合金成分及制备工艺与纳米晶复合永磁材料磁性能的关系,分析了稀土含量、添加合金化元素、快淬速度、晶化退火温度和时间对合金磁性能的影响机理,综述了纳米晶复合永磁材料开发研究的现状、存在问题及发展方向。     

铁基纳米晶合金的高频磁性能研究

分析了纳米晶Fe72.5Cu1Nb1.5Mo1.5V1Si13.5B9合金在Bm=0．05T～1．0T、f=20kHz～1000kHz范围内的铁损，以及f=0．5kHz～1000kHz范围内的幅值磁导率行为。主要阐述了铁损与f和Bm的关系，以及在固定的Bmf值情况下，给出铁损大小估算的关系式和合金的频散行为。     

纳米W、Cu固溶行为的研究

采用溶胶-喷雾干燥-热还原法制备了纳米W-Cu复合粉末,用日产3014-2Z型X射线自动衍射仪对W-Cu复合粉末和烧结坯进行物相分析。通过衍射图谱可以观察到经三步还原后的W-Cu复合粉末,存在W相和Cu相,通过精确测量各晶面所对应的衍射角,并采用标准Si校正,得到各种粉末和烧结坯中W相和Cu相的晶格常数,结果表明:这些数值与其分别对应的标准晶格常数0.3165 nm和0.3615 nm相比,出现一定偏差,说明W相和Cu相均存在一定的固溶,在烧结过程中,随着烧结温度的升高,W在Cu中的固溶度逐渐增大。     

Tensile Properties of TWIP Steel at High Strain Rate

 Tensile tests of TWIP steels of two compositions are performed in the strain rate range of 10-5-103 s-1. Results indicate that steel 1# does not exhibit TWIP effect but deformation induced martensitic transformation appears only. There exists TWIP effect in steel 3#. Tensile properties at room temperature are sensitive to strain rate in the studied strain rate ranges. Analysis on the relationship between strain hardening exponent and strain rates shows that strain induced martensitic transformation and formation of twins during deformation have significant influence on their strain hardening behavior.     

不同形变速率条件下低碳钢过冷奥氏体形变过程铁素体超细化

研究了低碳钢过冷奥氏体在760℃,形变速率为l s^-1和10 s^-1变形时组织演变规律.结果表明,形变速率为1 s^-1时真应力-应变曲线双峰特征为形变强化相变和铁索体动态再结晶的表征,相变形核集中在铁素体/奥氏体相界前沿奥氏体高畸变区,晶粒长大在时间和空间上受到限制,细化能力较高;形变速率提高到10 s^-1时,相变动力学提前,曲线只表现为形变强化相变的单峰特征,相变形核除了在上述铁素体/奥氏体相界前沿奥氏体高畸变区,还分布到奥氏体晶内各处,晶粒间约束有所减小,尺寸稍大.通过形变强化相变和铁素体动态再结晶可以获得平均晶粒尺寸为(1.98±1.07)μm和(2.33±1.01)μm(10 s^-1)左右的微细铁素体晶粒.     

Characterization of Dynamic Tensile Testing Using Aluminum Alloy 6061-T6 at Intermediate Strain Rates

Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydraulic machine at intermediate strain rates to validate the testing technique and to investigate the strain-rate effect on the material’s stress-strain behavior and failure mode. We present the experimental procedures and results discussing the constitutive response of the alloy at strain rates up to approximately 200??s-1. The predominant frequencies of the high-speed testing machine were characterized by modal analysis, and we analyzed the effect from vibration of the system and loading rate on flow stress by using a single degree-of-freedom (SDOF) spring-mass-damper model. We tested two different specimen sizes at a wide range of actuator velocities to achieve the desired strain rates. Results show that the yield strength, ultimate strength, and failure strain were dependent on strain rate. We fitted the data to the Johnson-Cook (JC) constitutive model, and the resulting parameters are comparable to published results in the literature. The fracture surface of specimens tested at different strain rates obtained by scanning electron microscopy (SEM) showed that the ductile failure mode was dominant for the alloy, and strain rates within the range examined affected the fracture morphology.     

Mechanical Behaviors of Ultrafine-Grained Ti-6Al-4V Alloy During Compression at Various Strain Rates

Metallurgical and Materials Transactions A - Ultrafine-grained (UFG) Ti-6Al-4V alloy with average grain sizes of 305, 430, and 669 nm was fabricated by high-energy ball-milling and spark plasma...     

Constitutive Equation for AISI4140 Steel Applicable to a Wide Range of Strain Rates at Elevated Temperatures

This paper presents a method to determine a constitutive equation of AISI4140 steel when it deforms at a wide range of strain rates (0.05~1900s⁻¹) under high temperatures. The fundamental idea considered here is that the dependency of flow stress upon strain rate and temperature is described by the power law model of the Arrhenius equation. Strengthening coefficient and work hardening exponent are introduced to take a simplified treatment to the flow stress behaviour characteristics without losing accuracy of the model. The validity of the proposed constitutive equation has been examined using results obtained from the hot compression test (GLEEBLE^® 3500C) and Split Hopkinson‐Pressure Bar test at high temperature. Results revealed the proposed constitutive equation could predict flow stress‐strain curve of the steel with an acceptable amount of error at the ranges of strain rates.     

Resolving the Martensitic Transformation in Q&P Steels In-Situ at Dynamic Strain Rates Using Synchrotron X-ray Diffraction

The dynamic deformation response of two quenching and partitioning (Q&P) steels was investigated using a high strain rate tension pressure bar and in-situ synchrotron radiography and diffraction. This allowed for concurrent measurements of the martensitic transformation, the elastic strains/stresses on the martensite and ferrite, and the bulk mechanical behavior. The steel with the greater fraction of ferrite exhibited greater ductility and lower strength, suggesting that dislocation slip in ferrite enhanced the deformability. Meanwhile, the kinetics of the martensitic transformation appeared similar for both steels, although the steel with a greater ferrite fraction retained more austenite in the neck after fracture.

     

Effect of Microstructure in TRIP Steel on Its Tensile Behavior at High Strain Rate

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高温热解法合成硫化亚铜纳米晶

以氯化铜和十二烷基硫醇为原料,聚乙二醇为表面活性剂,采用高温热解法在200℃反应30min,成功制备了硫化亚铜纳米碟晶体。利用X射线粉末衍射仪(XRD)、紫外可见吸收光谱(UV-3600)、透射电子显微镜(TEM)以及高分辨透射电子显微镜(HRTEM)、元素分析(EDS)等对产品进行表征。结果表明,所制备的硫化亚铜纳米晶体为六方晶相结构,吸收光谱显示吸收峰在1880nm左右,并且为宽带吸收,透射电镜表明硫化亚铜纳米碟晶体的直径约为80nm。光催化试验表明,在紫外和可见区该硫化铜纳米晶具有较高的光催化活性。经35W氙灯和450 W高压汞灯光照射30min,亚甲基蓝的降解率分别达到98.2%和81.6%。